Freshwater fish are those that spend some or all of their lives in
fresh water, such as
rivers and
lakes, with a
salinity of less than 0.05%. These environments differ from marine conditions in many ways, the most obvious being the difference in levels of salinity. To survive fresh water, the fish need a range of
physiologicaladaptations.

41.24% of all known species of
fish are found in fresh water. This is primarily due to the rapid
speciation that the scattered habitats make possible. When dealing with ponds and lakes, one might use the same basic models of speciation as when studying
island biogeography.

Contents

Physiology

Freshwater fish differ physiologically from salt water fish in several respects. Their
gills must be able to diffuse dissolved gasses while keeping the salts in the body fluids inside. Their scales reduce water diffusion through the skin: freshwater fish that have lost too many scales will die. They also have well developed
kidneys to reclaim salts from body fluids before excretion.

Migrating fish

Sturgeons are found both in anadromous and fresh water stationary forms

Many species of fish do reproduce in freshwater, but spend most of their adult lives in the sea. These are known as
anadromous fish, and include, for instance,
salmon,
trout, sea lamprey[1] and
three-spined stickleback. Some other kinds of fish are, on the contrary, born in salt water, but live most of or parts of their adult lives in fresh water; for instance the
eels. These are known as catadromous fish.

Species migrating between marine and fresh waters need adaptations for both environments; when in salt water they need to keep the bodily salt concentration on a level lower than the surroundings, and vice versa. Many species solve this problem by associating different habitats with different stages of life. Both eels, anadromous salmoniform fish and the
sea lamprey have different tolerances in salinity in different stages of their lives.

Classification in the United States

Among fishers in the United States, freshwater fish species are usually classified by the water temperature in which they survive. The water temperature affects the amount of oxygen available as cold water contains more oxygen than warm water.[2]

Warmwater

Warmwater fish species can survive in a wide range of conditions, preferring a water temperature around 80 °F (27 °C). Warmwater fish can survive cold winter temperatures in northern climates, but thrive in warmer water. Common warmwater fish include
catfish,
largemouth bass,
bluegill,
crappies, and many other species from the
Centrarchidae family.

Coolwater

Coolwater fish species prefer water temperature between the coldwater and warmwater species, around 60 to 80 °F (16–27 °C). They are found throughout North America except for the southern portions of the United States. Common coolwater species include
muskellunge,
northern pike,
walleye, and
yellow perch

Status

North America

About four in ten
North American freshwater fish are endangered, according to a pan-North American study, the main cause being human pollution. The number of fish species and subspecies to become endangered has risen from 40 to 61, since 1989.[3]